Mandelonitrile, structure

Several studies have tackled the structure of the diketopiperazine 1 in the solid state by spectroscopic and computational methods [38, 41, 42]. De Vries et al. studied the conformation of the diketopiperazine 1 by NMR in a mixture of benzene and mandelonitrile, thus mimicking reaction conditions [43]. North et al. observed that the diketopiperazine 1 catalyzes the air oxidation of benzaldehyde to benzoic acid in the presence of light [44]. In the latter study oxidation catalysis was interpreted to arise via a His-aldehyde aminol intermediate, common to both hydrocyanation and oxidation catalysis. It seems that the preferred conformation of 1 in the solid state resembles that of 1 in homogeneous solution, i.e. the phenyl substituent of Phe is folded over the diketopiperazine ring (H, Scheme 6.4). Several transition state models have been proposed. To date, it seems that the proposal by Hua et al. [45], modified by North [2a] (J, Scheme 6.4) best combines all the experimentally determined features. In this model, catalysis is effected by a diketopiperazine dimer and depends on the proton-relay properties of histidine (imidazole). R -OH represents the alcohol functionality of either a product cyanohydrin molecule or other hydroxylic components/additives. The close proximity of both R1-OH and the substrate aldehyde R2-CHO accounts for the stereochemical induction exerted by RfOH, and thus effects the asymmetric autocatalysis mentioned earlier. 

Early preparations of the product from benzil, hydrogen cyanide, and hydrogen chloride (315) and by the action of hydrogen chloride on mandelonitrile (benz-aldehyde cyanohydrin) (316) were formulated by Japp and Knox (317) as 3-hydroxy-2,5-diphenylpyrazine, and a mechanism for the reaction was proposed by Ingham (318). The structure of the product, however, has been refuted by Gallagher et al. (282). 

In a different approach the pH stability of AfHNL was improved by the introduction of a G-terminal bacterial flavin-based fluorescent protein (FbFP) fusion tag [172]. The increased half-life of cFbFP-AfHNL at acidic pH values (about a 43-fold increase at pH4.75) was attributed to a change in the quaternary structure in comparison to the wild-type enzyme. The fusion protein was investigated in the synthesis of mandelonitrile and 2-chloromandelonitrile in a two-phase system with MTBE. At pH4.75, (i )-mandelonitrile was produced with 75% 5tield and an ee>96% after 60 min. Maximum conversion of 2-chlorobenzaldehyde (78%) was achieved after 5 min with an ee of 99%. 

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